Water is a building's worst enemy. Whether it comes from precipitation, groundwater, or condensation, water can, over time, cause mold and mildew, rotting of wood structures, corrosion of metals, separation of paint from surfaces, spalling of masonry and concrete, and health problems for building occupants. Moisture problems are a principal factor limiting the useful service life of a building.
Groundwater can be shunted away by drains and water barriers; buildings can be sheltered from rainwater by roofs and walls that shed water; but condensation is particularly insidious because it originates within the building itself.
Whenever a building is heated or cooled, a danger exists that moisture-laden air may travel from the warmer side of an exterior wall to the colder side, condensing when it reaches any surface colder than its dewpoint.
FIG. 1 illustrates the condensation problem in a heating season 102. Moisture may be added to the air 101 inside a building by various sources, such as tub baths and showers, respiration and perspiration of pets and humans, humidifiers, cooking, dishwashing, internal clothes dryer venting, floor mopping, houseplants, and gas range pilot lights. A chimney effect within the building may cause a chronic exfiltration of air 103 from the upper part of the structure, through cracks and other imperfections in the wall. A temperature gradient exists within the wall, from the warm inside wall 105 through the insulation 100 to the cold outside wall 106. When the airflow reaches a surface that is below the air's dewpoint, condensation 104 occurs. The condensation can continue over long periods of time, resulting in a significant accumulation of liquid water within the wall assembly.
FIG. 2 illustrates the basic method that has historically been used to combat winter condensation. The basic rule is, “Put a vapor barrier on the warm side of the wall.” In FIG. 2, as in FIG. 1, a temperature gradient exists in winter 202 from the warm inside wall 205 through the insulation 200 to the cold outside wall 206. In FIG. 2, a vapor barrier 204 has been added, which prevents the humid inside air 203 from traveling into the wall. As a result the dewpoint of the air within the wall cavity is equal to the lower dewpoint of the dry outside air. Therefore no condensation occurs within the wall. Humid interior air 201 is in contact with the interior side of the vapor barrier, but the vapor barrier is warm because it is on the warm side of the wall. In particular, it is warmer than the dewpoint of the humid interior air, and so no condensation forms on the vapor barrier. The vapor barrier does not have to be perfect to be effective. It is sufficient if the vapor barrier is significantly less gas-permeable than the wall structures between the vapor barrier and the outside. When this requirement is met, the wall will dry to the outside, and so the dewpoint of the air within the wall cavity will be approximately the same as the dewpoint of the dry outside air. No condensation will form.
FIG. 3 depicts the problem for a building that is air-conditioned in a cooling season 302. The temperature gradient now runs in the opposite direction, from a warm outside wall 306 through the insulation 300 to a cold inside wall. Air 301 inside the building is dehumidified as well as cooled. A reverse chimney effect induces an infiltration 303 of warm humid air in the upper part of the structure, from the outside to the inside. When this air contacts materials colder than its dewpoint, condensation 304 accumulates.
FIG. 4 illustrates the basic method that is recommended for warm climates, to combat this condensation. Now the exterior temperature is higher, so the vapor barrier 404 is placed on the outside. The vapor barrier prevents high-dewpoint exterior air from traveling through the wall. As a result the dewpoint of the air within the wall is equal to the lower dewpoint of the dry inside air. Therefore no condensation occurs within the wall. Humid exterior air 403 is in contact with the vapor barrier 404, but does not condense because the vapor barrier is on the warmer side of the wall, and is at a higher temperature than the dewpoint of the outside air. Again, the vapor barrier does not have to be perfect, only significantly less gas-permeable than the structures of the wall between it and the interior.
The solution for a heated building is FIG. 2. The solution for an air-conditioned building is FIG. 4. But what about the usual case, where the building is heated at various times, and cooled at various other times? One possibility that presents itself (U.S. Pat. No. 5,027,572) is to put vapor barriers on both the inside and outside of the wall. But this does not solve the problem because the dewpoint of the air between the two barriers will be at some value intermediate between the dewpoint of the outside air and the dewpoint of the interior air, depending on the relative amount of leakage on the two sides. If either side of the wall is below this value, condensation will form on that side. Merely placing a vapor barrier on the warm side is not sufficient. It is also necessary that, at the same time that the vapor barrier is blocking humid air on the warm side, any moisture within the wall must be allowed to leave toward the dry side. Otherwise moisture can be trapped between the two vapor barriers, leading to condensation. In other words, the air inside the wall must be the air from the dry side, with its lower dewpoint. During the heating season this is the exterior air, and during the cooling season it is the interior air.
Before the advent of air conditioning, buildings only had to cope with being heated. Any exterior wall that had more ventilation to the exterior than to the interior, whether by design or by accident, was safe from condensation. Most buildings today in temperate zones will be cooled in the summer and heated in the winter, and so will face the quandary described above. Indeed, buildings that are retrofitted with air conditioning commonly develop condensation problems as a result. Buildings that have survived for decades, or even centuries, may be destroyed when air conditioning is installed, by rotting of their structural wood members.
Various other proposals have been made to deal with the problem. US-2003/0205129, US-2004/0211315, and U.S. Pat. No. 6,793,713 propose periodically placing desiccant within the insulation cavity. US-2010/0233460 and US-2010/0229498 propose ventilating an insulating cavity with manually operated valves. US-2007/0094964, US-2007/0084139, and U.S. Pat. No. 7,247,090 describe systems with a dehumidifier that forces dehumidified air into the insulating cavity.
What is needed is an inexpensive insulating system that automatically, throughout all seasons, ventilates to the colder side, while blocking ventilation to the warmer side.
A component, that will be used in the current invention, and that is well known in the art, is a one-way valve that operates with low-pressure differential between inlet and outlet. U.S. Pat. No. 8,464,715 describes one-way valves that are used in non-rebreathing facemasks. US-3993096 describes a one-way valve operated by air pressure, used in air conditioners. U.S. Pat. No. 4,565,214 describes a flapper check valve that is operated by a low-pressure differential. U.S. Pat. No. 6,210,266 describes a flap valve for pressure relief in an automobile passenger compartment.
The requirements for a one-way valve in the current invention are that it be durable, and operate in response to a low-pressure differential between its inlet and outlet. It need not perfectly seal against wrong-way flow, but only restrict wrong-way flow to be significantly lower than right-way flow. It does not need to have a large flow rate, only a flow rate that is larger than whatever leakage exists in the vapor barriers of the current invention.
FIGS. 5, 6, 7, and 8 depict a low-pressure differential one-way valve 500, as is well known in the art. 502 is the inlet, and 501 is the outlet. A lightweight and flexible but strong membrane 504 in the shape of a disc is secured at its periphery 505 to be held above a barrier 506 with inlet holes 507. When inlet 502 pressure is higher than outlet 501 pressure (FIGS. 5 and 6), a flow 503 is established through the inlet holes 507 and out through the outlet 501. The schematic symbol depicting the flow condition is shown in FIG. 6. When inlet 502 pressure is lower than outlet 501 pressure (FIGS. 7 and 8), the membrane 504 presses against the barrier 506, blocking the holes 507 and preventing backflow (703). The schematic symbol depicting the non-flow condition is shown in FIG. 8.